The present invention relates to an airway adaptor with a sample port, and, more particularly, to an airway adaptor with a sample port which includes an air collector with multiple inlets.
For purposes of description, the discussion herein is focused on airway adaptors for use with human patients, it being understood that the present invention is not limited in scope only to use with patients and can beneficially be used in various other contexts.
Airway adaptors are generally used to collect gas samples for analysis, particularly from the exhaled breath of hospitalized patients who require a breathing apparatus, such as patients under anesthesia or those patients on life support systems. Typically, an endotracheal tube extends from such a patient to a breathing apparatus, carrying gases to the patient and the exhaled breath of the patient away from the patient. The adaptor connects the section of the endotracheal tube leading from the patient to the section leading to the breathing apparatus. The adaptor may be in the shape of a "T", such that these adaptors are also known as "T-pieces". The top or cross-piece of the "T" is a tube, through which gases travel to the patient, and through which the exhaled breath returns from the patient. The other part of the "T" is a port which projects from the wall of the tube, and is used to collect samples of the gas flowing through the tube. Alternatively, the adaptor may be in the shape of an elbow, such that this type of adaptor is also called an "elbow piece". The tube in these adaptors forms the elbow, and the port again projects from the wall of the tube.
In either type of adaptor, the port is connected at its other end to a gas analyzer. A sidestream of the patient's exhaled breath flows through the port, to the gas analyzer to be analyzed. The results of this non-invasive analysis provide an indication of the patient's condition, such as the state of the patient's pulmonary perfusion, respiratory system and metabolism.
The accuracy of this non-invasive analysis of exhaled gases depends on the ability of a sampling system to move a gas sample from the patient to the gas analyzer while maintaining a smooth, laminar flow of gases, such that there are as few alterations to the waveform and response time of the gases as possible. The waveform of the gas is critical for accurate analysis. As gas travels from the patient to the gas analyzer, it moves in a wave. The composition of gases changes throughout this wave, defining the waveform. These changes can occur within 10-100 msec, and give important information about the condition of the patient. Internal mixing of the gas sample, or alterations in the waveform, reduces the accuracy of the analysis of the sample by the gas analyzer, and reduces the amount of information obtained from that analysis.
Internal mixing of the gas sample and alterations in the waveform both slow the response time. As each gas travels in a wave, the wave has peaks, or high concentrations of gas. The response time is the time elapsed between the appearance of the base of a peak and the appearance of the peak itself. A fast response time indicates that the peaks are relatively tall and narrow, and that the peaks have not broadened since the gas was exhaled from the patient. Since the accuracy of the gas analysis, and the information obtained from that analysis, depend upon the waveform remaining substantially unaltered, a fast response time is desirable.
Merely stabilizing one of these factors is not sufficient for the accurate analysis of a gas sample. Alterations in one of these factors tend to affect the other factors, multiplying the changes to the gas sample, and exponentially reducing both the accuracy of the gas analysis, and the amount of information obtained from that analysis. For example, mixing of the gas tends to slow the response time. Thus, it is crucial that the airway adaptor alters these factors as little as possible.
A significant obstacle to preventing these alterations to the gases, and hence to obtaining an accurate gas analysis, is that the exhaled breath of such patients frequently contains substances which can block or clog the sampling apparatus, such as liquid or solid secretions, or mixtures thereof, including mucous, saliva and condensed water. Therefore, the airway adaptor must include means for separating the desired exhaled gases from these solids, liquids, or mixtures thereof. These separating means are placed in either the tube or the port of the airway adaptor. However, such means are also subject to blocking or clogging, which can reduce the pressure of gases traveling through the airway adaptor to the gas analyzer. Such a pressure drop may cause numerous alterations to the gas sample, including alteration of the waveform, mixing of the gas, and alterations in gas concentration, all of which reduce the accuracy of the gas analysis, and the amount of information obtained from that analysis. The concentration of the gas is particularly affected by changes in pressure, since gas concentration is directly dependent on the pressure of the gas, and is usually presented in units of millimeters of mercury. Hereinafter the term "pressure drop" refers to a decrease in the pressure exerted by the gas itself.
The need for an accurate analysis of the gas, as well as the overall demands of human gas analysis, dictate the required features of an airway adaptor. First, the gases should be separated from the liquids, solids or mixtures thereof, while maintaining a smooth, laminar flow of gases, and without a production of substantial pressure drops, or an alteration in the gas waveform. Second, minimal added void volume should be present in the adaptor or sample port, which might cause mixing of gases. These characteristics are critical for accurate sample analysis and for obtaining maximum information from the analysis, since mixing of gases, disruption of the smooth, laminar flow of gases, or alterations in the waveform of the gases can produce significantly inaccurate results, as described above. Furthermore, pressure drops tend to exacerbate gas mixing, reductions in gas flow rate, and alterations of gas waveform, and should therefore be avoided.
Further features are dictated by the demands of human gas analysis. The airway adaptor employing a means of separation should be low maintenance; that is, it should not require frequent cleaning or replacement. Also, the airway adaptor should be easy to use. Unfortunately, currently available adaptors have serious flaws. These less viable adaptors can be easily distinguished from the present invention, which successfully meets the requirements.
These previously known adaptors have often included a filter in the sample port for separating gases from liquids, solids and combinations thereof. Other such adaptors have filters or baffles in the tube of the airway adaptor. However, none of these filter-based constructions solves the inherent tendency of hydrophobic, porous materials to substantially increase the pressure drop of the gas as it crosses the filter, interfering with the waveform and reducing the accuracy of the sample analysis. Furthermore, such a pressure drop tends to increase over time, as patient secretions and condensed water collect on or in the filter. For example, a simple flat filter with a relatively small surface area, such as that disclosed in U.S. Pat. No. 4,456,014 to Buck et al., is easily covered with patient secretions or condensed water, which accelerates this drop in pressure.
Increasing the diameter of the filter so that it is larger than the sample port diameter, such as in U.S. Pat. No. 4,679,573 to Parnoff et al. (hereinafter referred to as "Parnoff"), reduces the rate at which such a filter may become blocked. However, in order to avoid adding void volume to the sampling apparatus, the design of the Parnoff airway adaptor has the filter lying against the tube wall, which increases the tendency of the filter to become covered with condensed water or patient secretions.
The surface area of the filter can be increased without increasing the diameter, if the shape of the filter is altered from flat to dome-like or conical, as described in both Parnoff and PCT Application No. US 90/04353 to Wo. However, the dome must have thick walls in order to maintain its shape under pressure, which encourages the mixing of gases, since the flow rate is sharply reduced by the thickness of the walls. Furthermore, the thickness of the walls adds void volume to the sample apparatus. Finally, the filter membrane must have a small pore size to prevent the entry of condensed water and other liquids, which further interferes with the smooth, laminar flow of gases, and increases the pressure drop.
A filter or baffle can be added to the tube of the airway adaptor itself, rather than to the sample port, which may reduce added void volume. For example, an inner lining of the airway adaptor itself can be made permeable only to gas, so that gas escapes to the sample tube, while liquids and solids are trapped in the adaptor. In this sense, the tube itself is the filter, as described in U.S. Pat. No. 4,985,055 to Thorne et al. Alternatively, a baffle may be placed in the tube of the adaptor, rather than a filter, as described in U.S. Pat. No. 4,558,708 to Labuda et al. However, condensed water which collects on the walls of the tube of the airway adaptor quickly fills this type of baffle, after which the baffle is no longer effective. Filled baffles are heavy, and tend to put a strain on the connections between the airway adaptor and the endotracheal tubing. These baffles also introduce further amounts of void volume. Furthermore, neither configuration solves the inherent tendency of filters and baffles to become blocked. Indeed, these adaptors may themselves become blocked by liquid or solid material, with a potentially adverse effect on the patient.
Alternatively, a backflush device can be used to remove liquid or solid material which is blocking the filter, as in U.S. Pat. No. 5.042,522 to Corenman et al. However, such a device still does not solve the problem of the decrease in gas pressure as the gas crosses the filter, nor the related problem of slow response times.
Clearly, filters are not an adequate solution due to the inherent flaws in their performance. It is known in the literature to construct an adaptor which does not rely on filters for separating gases from liquids, solids or mixtures thereof. E.P.C. No. 0275105 to Spacelabs, Inc. (hereinafter referred to as "Spacelabs") describes an adaptor which does not use any kind of filter or filter-like device. The tube of the adaptor of Spacelabs has two chambers, connected by radial channels extending from the inner chamber to a chamber which is in the form of an annular channel. This annular channel is formed in a section of the adaptor with a constricted diameter. The gases flow from the inner chamber through the radial channels to the annular channel, and from the annular channel to the port.
The adaptor of Spacelabs is not an adequate solution to the problems above. Airflow through the airway adaptor may be compromised, since the diameter of the adaptor is constricted. Furthermore, the port of the adaptor of Spacelabs will tend to suck in water or other liquids if the port is not kept upright. Keeping the port upright is unrealistic in a hospital environment, since the adaptor may be incorrectly installed by hospital staff, and since patients may move, which can cause the tube and the port to rotate, requiring frequent repositioning of the adaptor.
The present invention also does not rely upon a filter to separate gases from liquids, solids or mixtures thereof. Thus, the present invention is not subject to the inherent flaws of adaptors which rely upon filters. Furthermore, the present invention can be easily distinguished from, and is greatly superior to, the adaptor of Spacelabs, since the present invention alters the structure of the port of the airway adaptor, rather than of the entire adaptor. The internal structure of the adaptor of the present invention is completely different from that of the adaptor of Spacelabs, and does not compromise airflow through the adaptor, nor does the port of the present invention need to be kept upright for optimum efficacy. Thus, the present invention is easier to install and to maintain, particularly since the airway adaptor of the present invention can be freely rotated in a variety of orientations, and still maintain its efficiency.
Thus, none of the above previously known configurations successfully fulfills the criteria for an airway adaptor listed above. The present invention does fulfill these criteria successfully, in a form which is clearly and easily distinguishable from the above previously known configurations.
There is thus a widely recognized need for, and it would be highly advantageous to have, an airway adaptor which does not reduce gas pressure or flow rate, or alter the gas waveform, which does not easily become blocked or clogged, which has minimal added void volume, yet which is easy to use and does not require frequent repositioning or maintenance, and which is freely rotatable, so that it is efficient in a variety of orientations.